The incident occurred at the Western Forest Products mill. When police arrived, shortly after 7 a.m. local time, they arrested a suspect believed to be the shooter. Police said four people were hospitalized, but did not give names or details.
A spokeswoman for Island Health Authority said one victim, at Victoria General Hospital, is in critical condition.
A witness said a man entered the workplace with a shotgun and a handgun, and wounded a woman and two men.
Pat Kinney, a spokesman for the United Steel Workers Union, which represents employees at the lumber mill, confirmed the shooting occurred at the work site. A worker said employees had been sent home for the day.